Fluorescent substances are generally a light emitting material emitting light with a unique wavelength by absorbing energy in the form of light or electricity from the outside, and may be divided into inorganic fluorescent substances, organic fluorescent dyes, nanocrystal fluorescent substances and the like depending on the components forming the fluorescent substance and light emission mechanism.
Recently, attempts to modify a spectrum of a light source using such fluorescent substances have been diversely made. This relates to a fluorescent substance absorbing some of specific wavelengths of light coming out of a light source, converting this to light with a longer wavelength in a visible region and emitting the light, and depending on light emission properties of the fluorescent substance, brightness, color purity, color gamut and the like of the emitted light may be greatly enhanced.
An inorganic fluorescent substance is formed with a parent such as a sulfide, an oxide or a nitride, and activator ions, and may be used in high-quality display apparatuses since physical and chemical stability is excellent and high color purity is reproduced, however, there are disadvantages in that the fluorescent substance is very high-priced, has low light emission efficiency, and particularly, development of the fluorescent substance excited and emitting light in a near ultraviolet or blue region of 400 nm or higher is limited.
In a nanocrystal fluorescent substance formed with groups II to IV or groups III to V semiconductor particles having a few nanometer size, the fluorescence wavelength becomes different depending on the particle size unlike an organic fluorescent dye, and as the particle size decreases, light with a shorter wavelength emits, and a visible region having a target wavelength may all be expressed by controlling the size. In addition, the nanocrystal fluorescent substance has a larger extinction coefficient than general organic dyes by 100 times to 1000 times and also has high quantum efficiency, and thereby generates very strong fluorescence. Particularly, the nanocrystal fluorescent substance observes only a transition from a ground vibration state of a conduction band to a ground vibration state of a valence band, and as a result, the fluorescence wavelength mostly shows monochromatic light. However, there are disadvantages in that high raw material costs make it difficult to secure price competitiveness, and the fluorescent substance is weak against heat or oxygen.
On the contrary, an organic fluorescent dye has diverse light emission spectra compared to the inorganic fluorescent substances, has excellent quantum efficiency, and particularly has an advantage of being low-priced, and therefore, is well worth being used in a light conversion device. However, for increasing conversion efficiency of light passing through the organic fluorescent dye and intensity of the converted light, the concentration needs to be increased, and in this case, extinction caused by the concentration may not be avoided, and particularly, stability against heat or light is known to decrease.